Tundra is an arctic biome where it is too cold for trees to grow.
Outside of the polar ice caps, tundra has the coldest temperatures
on Earth. There are two types of tundra: arctic tundra, which is
also found in Antarctica, and alpine tundra, which is found only at
high altitudes. Arctic tundra occurs north of the arctic circle and
south of the antarctic circle. It covers much of Alaska and vast
areas of northern Canada and Russia. It is also found along the
northern coast of Antarctica. Alpine tundra occurs in mountains
around the world at any latitude, but only above the tree line. The
tree line is the edge of the zone at which trees are able to
survive. Alpine tundra is found in the Rocky Mountains in the
United States and in several other mountain ranges around the
world. Both types of tundra receive very low precipitation, but
little of it evaporates because of the cold. Arctic tundra has
permafrost, which is soil that is frozen year-round. The top layer
of soil thaws in the summer, but deeper layers do not. As a result,
water cannot soak into the ground. This leaves the soil soggy and
creates many bogs, lakes, and streams. Alpine tundra does not have
permafrost, except at very high altitudes. Therefore, alpine tundra
soil tends to be dry rather than soggy. Global warming poses a
serious threat to Arctic tundra biomes because it is causing the
permafrost to melt. When permafrost melts, it not only changes the
tundra. It also releases large amounts of methane and carbon
dioxide into the atmosphere. Both are greenhouse gases, which
contribute to greater global warming. The most common vegetation in
tundra is mosses and lichens. They can grow in very little soil and
become dormant during the winter. Tundra is too cold for amphibians
or reptiles, which cannot regulate their own body heat. Insects
such as mosquitoes can survive the winter as pupae and are very
numerous in summer. In addition, many species of birds and large
herds of caribou migrate to arctic tundra each summer. However, few
birds and mammals live there year-round. Those that remain have
adapted to the extreme cold. Polar bears are an example. They have
very thick fur to insulate them from the cold. In alpine tundra,
animals must adapt to rugged terrain as well as to cold. Alpine
animals include mountain goats, which not only have wool to keep
them warm but are also sure-footed and agile.
